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Strength - Cost
General Information
Physical Insights
Example Uses
Simple Questions
Further Questions
This chart helps for choosing strong materials at low cost.
General Information
Strength measures the resistance of a material to failure, given by the applied stress (or load per unit area) .
The chart shows yield strength in tension for all materials, except for ceramics for which compressive strength is shown (their tensile strength being much lower).
Many applications require strong materials, e.g. screwdrivers, safety belts - these lie at the top of the chart.
Unfortunately there are few cheap high strength materials (top left).
The 'hardness' of a material is often used to indicate things like wear resistance. Hardness is
measured by making a small surface indentation. It is not a material property, but is closely
related to the strength, so strength gives a good indicator of wear resistance.
Physical Insights
Ceramics and glasses have directional covalent bonds. They are weak in tension because they are sensitive to small cracks or flaws.
Metals do not have directional bonds and have similar tensile and compressive properties.
Cellulose microfibres make wood strong in tension along the grain.
Alloys are much stronger than pure metals.
Metals can be strengthened by heat treating to change the microstructure.
Example Uses
Teflon as a non stick surface for frying pans.
Ceramic for fire bricks and for coatings for jet engine blades.
Tungsten for light bulb filaments.
Simple Questions
Why is wood used for beams rather than stone, when stone has a higher strength?
When beams are bent one side is in tension, the other compression. Stone is not sufficiently strong in tension to be used for beams. Wood on the other hand has strong cellulose fibres which make it good in tension.
Why is concrete weak in tension?
Concrete is weak because it contains lots of small holes (flaws) – these can be seen under a low power microscope. Engineering ceramics are stronger because they do not contain large flaws.
Why is steel the most commonly used ‘strong’ material?
Steel is perhaps the cheapest high strength material. In addition is easy to process, either by casting or hot metal working. Its properties can be optimised by alloy composition and heat treatment.
Select materials for a screwdriver.
Fairly mild steel: very hard steels are too brittle and could crack.
Select materials for a glass-cutting tool.
High hardness (which is related to strength) is needed and toughness is not particularly important so that a diamond tip is best.
Further Questions
Why can polymers operate only at low temperatures when their polymer chains contain covalent bonds like ceramics?
What is the operating temperature of a light bulb filament?
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